Seeds brokers react as they look at the main board at the trading floor of the Buenos Aires Grain Exchange, in the Argentine capital on January 23, 2014. / LEO LA VALLE AFP/Getty Images

by Matt Krantz, USA TODAY

by Matt Krantz, USA TODAY

The idea of a full-blown stock market correction is looking less far-fetched, as fears about a slowdown in China are sending investors scurrying.

Sitting on big stock gains, investors were on the lookout for any signs of a correction and took the opportunity to sell. The Dow Jones industrial average Friday fell 318.24 points, or 2.0%, to 15,879. Added to Thursday's 217-point loss in the Dow, the much-watched measure of the stock market is now down 4% from its recent high notched at the end of last year.

Stocks suffered their worst two-day sell-off since June 20, 2013. Measured by the broad Wilshire 5000, stocks fell 3.2% during the week, their biggest weekly percentage loss since June 1, 2012.

Investors are now wondering if this is just the start of what could turn into a full-blown correction, unofficially defined as a fall of 10% or more. Given the suddenness of the move and the fact stocks soared so much last year, Hugh Johnson of Hugh Johnson Advisors says there's more pain to come. "Do we have further to go down? Yes," he says.

Investors are getting increasingly concerned about the stock market's action because of the:

â?¢ Bad history of Asian contagion. Investors' concern of a slowdown of the economy in China is a primary millstone for the market. Investors have some bad memories of how problems in Asia can spread, says Robert Maltbie of Millennium Asset Management. The so-called Asian Contagion of 1997 started in Thailand, but quickly rippled beyond. "Can these little countries tip the world into oblivion? No," says Maltbie. "But, they can cause volatility."

â?¢ Negative and sudden downward trajectory. Investors have swiftly shifted from being over-excited about stocks to starting to have major reservations. One measure of investors' nervousness, the Chicago Board Options Exchange's Market Volatility index, soared 30% Friday as investors hunkered down. It's very possible for the market to test its average level over the past 200 days, which for the Standard & Poor's 500, would be another 5.6% decline, says Ken Winans of Winans Investments.

â?¢ Importance of Januaries. Traders like to look at January as an early-warning system for how the year for stocks will do. So far, the S&P 500 is down 3.1%, hardly a rip-roaring start. When stocks fell in January, since 1936 on average, stocks have fallen as much as 18% during the year, Winans says. "We should be taking this seriously," he says.

â?¢ Disappointing earnings. According to S&P Capital IQ, 100 companies have provided earnings guidance for the fourth quarter of 2013. Of those, 80 are negative, 10 positive and 10 in line for a higher negative-to-positive ratio than the 15-year average.

Despite all the fear and concerns stocks could fall more, investors should stand ready to jump on buying opportunities, says Doug Sandler of RiverFront Investment Group. Much of the selling is simply a normal correction some forgot is a normal part of investing, he says. The fundamentals of the economy, including loan growth and corporate earnings remain sound, he says. "We knew a pullback was coming, we got ahead of ourselves," he says. "The bones of the market are in pretty good shape."

Dow Jones Industrial Average closed Friday at 15,879.11

This week: down 579.45 points or 3.52% and 318.24 points, 1.96%, for the day

Largest one-week point decline since the week ending Sept. 23, 2011. Largest one-week percentage decline since the week ending Nov. 25, 2011. Down three of the past four weeks

--Largest one-day point and percentage decline since June 20, 2013--Down four consecutive trading days. --Down 579.45 points or 3.52% over the last four trading days --Largest four-day points and percentage decline since June 24, 2013. --Longest losing streak since Jan. 13. 2014 - when the market fell four consecutive trading days. --Three of the 30 component stocks rose, 27 fell. --Dow is off 4.21% from its record high of 16,576.66 on Dec. 31, 2013--Up 142.54% from its "bear market" low of 6547.05 on March 9, 2009.--Lowest closing value since Dec. 17, 2013.